The present invention relates to micromachined devices, and more particularly to microstructures usable, for example, in vacuum microdevices.
Vacuum amplifying and switching devices offer several advantages over semiconductor devices for some applications. A vacuum itself cannot incur radiation damage, and vacuum emitters and collectors are relatively insensitive to radiative flux. Therefore, vacuum tubes may be employed in high-radiation environments, such as in nuclear-reactors, space, or other places where transistors may be destroyed. Also, unlike semiconductors (which suffer mobility degradation at elevated temperature), the performance of vacuum devices is not degraded in high temperature environments, such as oil and geothermal wells, engines, and nuclear reactors. Finally, due to the higher maximum velocity of electrons in a vacuum (order of 108 m/s versus order of 10.sup.6 m/s in semiconductors), vacuum devices can potentially perform at higher frequencies.
The present invention mitigates the thermal problems which can be expected to arise in microfabricated thermionic-emission devices. These problems include heat loss and overheating of nearby devices. Because of such problems, researchers have focussed on non-thermionic field-emission vacuum devices, which typically have sharply pointed IC-processed cathodes and closely spaced grids. See, for example, Technical Program of Third International Vacuum Microelectronics Conference, IEEE, Monterey, Calif., July 1990. Anodes for these structures are usually added later mechanically, rather than by microfabrication techniques, making the devices more expensive to produce.
A smaller number of thermionic-emission devices with at least some of the electrodes on a single chip have also been fabricated. In the thermionic devices known heretofore, the entire substrate had to be heated and maintained in an evacuated enclosure. This, however, severely limits the systems that can be used with such devices since only devices which are functional at very high temperatures can be integrated on the substrate. See D. V. Geppert et al., "Low-temperature thermionic emitter," Interim Scientific Report (SRI Project PYU-7147), Stanford Research Institute, Menlo Park, Calif., May 19, 1969; and D. K. Lynn et al., "Thermionic integrated circuits: electronics for hostile environments," IEEE Transactions on Nuclear Science, Vol. 32, No. 6 (Dec. 1985), pp. 3996-4000.
Exposed suspended filaments have been fabricated for various uses including incandescent lamps. Due to the fact that, unlike the filaments of the present invention, these filaments are exposed, they have low power efficiency and limited device life. See, eg, U.S. Pat. No. 4,724,356, issued Feb. 9, 1988.
Empty sealed cavities consisting of a recess in a substrate and a sealing membrane along the surface of the substrate have been fabricated and are discussed in S. Sugiyama et al., "Micro-diaphragm pressure sensor," Tech. Digest, IEEE International Electron Devices Meeting, pp. 184-187, 1986.
Devices comprising exposed micromachined moving parts have been fabricated. In many cases, it would be advantageous to seal them to eliminate friction due to gases or liquids they may be immersed in, or damage from reactive or contaminating ambients.
In view of the foregoing, an object of the present invention is to provide a microstructure where the magnitude or the variability of friction, thermal conduction, chemical attack, or contamination to which a device is exposed is reduced.
It is a more specific object of the present invention to provide a microstructure where the magnitude or the variability of friction, thermal conduction, chemical attack, or contamination to which a device is exposed is reduced, wherein the device can be fabricated by conventional IC fabrication technology.
Yet another object of the present invention is to provide integrated circuits combining hot thermionic-emission device components with other devices which require lower temperatures and providing metallurgically compatible surroundings for the hot elements.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.